Ant-Built Oasis Protects Colonies Through Thailand’s Dry Season

Tropical ants often struggle through the dry season, when high temperatures, falling humidity and scarce insect prey can reduce foraging and interrupt colony growth. A study of the plant ant Cladomyrma sirindhornae in Thailand reveals a striking exception: colonies living inside the hollow vines of Sphenodesme pentandra maintain large worker populations and continue producing brood through both wet and dry periods. The host plant appears to function as an ecological refuge, supplying protected nesting space and a dependable food supply that buffers colonies against seasonal stress.

Researchers examined 243 vines collected from separate host trees in secondary forests, forest edges and forest paths across central and eastern Thailand. Nearly all the vines contained naturally hollow stems, but the ants themselves did not appear to excavate these spaces. Cladomyrma sirindhornae occupied 223 vines, or 91.8 percent, while 11 vines housed other ant species and nine were unoccupied. This exceptionally high occupation rate indicates a close association between the ant and S. pentandra, which is currently the only known host of C. sirindhornae.

The partnership unfolds on an impressive architectural scale. The sampled vines averaged 16.9 metres in length and carried an average of 97 small, elliptical entrance holes. C. sirindhornae colonies occupied cavities from the base of the stem to the canopy, using a network of pre-existing chambers rather than digging its own tunnels. Major workers defended many entrances by blocking them with their heads, a defensive behaviour known as phragmosis. Dealate queens were usually found in swollen portions of the stem, between three and 12 metres above the base.

Inside the domatia, the researchers also found soft, light-orange, gel-like food bodies distributed throughout the cavities. Workers repeatedly carried these structures, indicating that they are an active food resource, although no extrafloral nectaries were found in the plant. Colonies also cohabited with pseudococcid scale insects, which produce honeydew. Both resources could help sustain the ants when external prey is scarce, although honeydew alone is relatively nitrogen-poor and may require complementary food sources to support colony growth.

To determine whether this sheltered lifestyle protected colonies from the dry season, researchers dissected 20 mature nests, with ten collected during each season. These colonies averaged about 5,349 workers and had a notable division of labour. Major workers, distinguished by their broad heads, made up roughly 30 percent of the workforce, while smaller minor workers formed the majority. Major workers ranged from 0.91 to 1.00 millimetres in head width, comparable to alate queens, whereas minor workers measured only 0.60 to 0.70 millimetres. Their smaller heads allowed them to pass through entrances that major workers could seal from outside, suggesting a precisely coordinated defence system.

Colony size did not vary significantly between wet and dry seasons. A Poisson generalized linear mixed model found no statistically significant effect of season on the number of workers, while host-plant length was also not a significant predictor. Total major and minor workers numbered 5,164 on average in the wet season and 5,283 in the dry season. The relative abundance of worker pupae, larvae and scale insects also did not differ significantly between seasons. In the dry-season colonies studied alongside these plant ants, researchers observed that many ground-dwelling ants had little or no brood, emphasizing how unusual it was for the protected Cladomyrma colonies to remain so active.

Seasonal differences emerged in reproduction rather than basic colony growth. Colonies produced both workers and sexual offspring in each season, but reproductive pupae and the combined number of alate queens and males were more abundant in the wet season. The relative number of reproductive pupae differed significantly between seasons, as did the combined reproductive-adult count. By contrast, separate comparisons of alate queens and males did not reach statistical significance, indicating that the clearest pattern emerged when the two sexes were considered together. These results suggest that resource demand for sexual development and dispersal rises when environmental conditions are less favorable.

The seasonal timing was also visible during colony founding. Newly established nests were encountered more frequently in the wet season. A founding queen chewed into a young stem just below a node, creating a small “primary hole” that she then blocked with her head. Continued growth of the plant eventually closed the opening, while the affected node became swollen. These early colonies generally contained a single dealate queen, a small quantity of brood and nanitic workers. Because winged reproductives must leave the host to mate and establish new nests, moderate temperatures, high humidity and other favorable outdoor conditions may be more important for dispersal than for the day-to-day persistence of mature colonies.

Queen physiology offers a possible explanation for the scale of these colonies. Thirteen queenright nests were examined, revealing highly physogastric queens with an average of 32 ovarioles, dense yellow bodies and numerous developing oocytes. The number of developing oocytes did not differ significantly between seasons, and major and minor workers each possessed only two ovarioles. Although roughly 32 ovarioles may appear modest for a colony capable of housing thousands of workers, the queens’ enlarged reproductive bodies and abundant mature eggs suggest substantial fecundity. Seven dissected colonies lacked a dealate queen, but workers in queenright and queenless nests did not differ significantly in number; the researchers noted that a queen might escape when a lower stem section was cut during collection.

Taken together, the findings portray S. pentandra as an oasis within the seasonally harsh forest floor. Its hollow stems offer buffered humidity, while food bodies, scale-insect honeydew and potentially other internal resources reduce dependence on exposed foraging. C. sirindhornae in turn may benefit its pioneer host by discouraging herbivores, limiting plant competitors or contributing nutrients through accumulated waste, although those benefits remain to be measured directly. The ants’ high occupation of the vine, coupled with the stable size of mature colonies, suggests a highly specialized mutualism in which the plant’s architecture protects a resident insect population. The central seasonal signal is not a boom-and-bust in colony size, but a shift in reproductive strategy: growth continues year-round inside the host, while mating and new colony establishment are concentrated in the wet season.

Subject of Research: Seasonal colony dynamics and reproductive behavior of the plant ant Cladomyrma sirindhornae in its Thai host plant Sphenodesme pentandra.

Article Title: Ant-plants as oases: Colonies of Cladomyrma sirindhornae ( Hymenoptera , Formicidae ) thrive in their host plants Sphenodesme pentandra in both the wet and dry seasons

Article References: Khachonpisitsak, S., Jaitrong, W., Jeenthong, T., Wongkunanusorn, C., Laedprathom, K., Chaianunporn, T., Khalife, A., Aupanun, S., & Ito, F. (2025). Ant-plants as oases: Colonies of Cladomyrma sirindhornae (Hymenoptera, Formicidae) thrive in their host plants Sphenodesme pentandra in both the wet and dry seasons. Journal of Hymenoptera Research, 98, 559-577. https://doi.org/10.3897/jhr.98.144518

Image Credits: AI Generated

DOI: 10.3897/jhr.98.144518

Keywords: Cladomyrma sirindhornae, Sphenodesme pentandra, ant-plant mutualism, myrmecophytes, seasonal ecology, Thailand, colony reproduction, scale insects, dry-season refuge, Ant-plants, oases, Colonies